A promising route in the search for renewable energy sources is using solar energy to split water into oxygen and hydrogen. This process is environmentally “clean”, i.e., does not produce greenhouse gases. Other advantages include earth's large water reservoir and the high energy density of hydrogen fuel.
Solar energy conversion into hydrogen can be accomplished in a photoelectrochemical (PEC) cell, where solar energy is absorbed at the semiconductor photoanode and/or photocathode. In a PEC cell including a photoanode, as a result of absorbing photons, excited electrons transport through the circuit and arrive at the opposing cathode where they take part in the half-cell reaction of hydrogen reduction. Holes transport in the opposite direction toward the photoanode surface and complete the other half-cell reaction of water oxidation. Overall, this process splits water to produce hydrogen and oxygen gas.
Conversion efficiencies of PEC cells are limited by the intrinsic absorbing qualities of the electrode. The electrode should be a good conductor, a good solar light absorber, a good catalyst, and have an appropriate band edge alignment for water splitting. Furthermore, the electrode should be relatively inexpensive, abundant, nontoxic, and maintain stability in operating conditions. Most of said qualities can be found in hematite (α-Fe2O3, also termed herein “Fe2O3”). Hematite has been widely studied for use in PEC cells [Engel, J.; Tuller, H. L., Physical Chemistry Chemical Physics 2014, 16, 11374-11380; Yang, Y.; Ratner, M. A.; Schatz, G. C., The Journal of Physical Chemistry C 2013, 117, 21706-21717; Yang, Y.; Ratner, M. A.; Schatz, G. C., The Journal of Physical Chemistry C 2014, 118, 29196-29208; Pu, A.; Deng, J.; Li, M.; Gao, J.; Zhang, H.; Hao, Y.; Zhong, J.; Sun, X., Journal of Materials Chemistry A 2014, 2, 2491-2497]. However, Fe2O3 also has disadvantages, including low charge mobility, high electron-hole recombination rate, and a large overpotential of 0.5-0.6V that is required for water oxidation [Sivula, K.; Le Formal, F.; Gratzel, M., Chem Sus Chem 2011, 4, 432-449].
The efficiency of a PEC cell containing Fe2O3 can be amplified through doping [Liao, P.; Keith, J. A.; Carter, E. A., JACS 2012, 134, 13296-13309; Huda, M. N.; Walsh, A.; Yan, Y.; Wei, S.-H.; Al-Jassim, M. M., Journal of Applied Physics 2010, 107, 123712-123712-6; Kleiman-Shwarsctein, A.; Huda, M. N.; Walsh, A.; Yan, Y.; Stucky, G. D.; Hu, Y.-S.; Al-Jassim, M. M.; McFarland, E. W., Chemistry of Materials 2009, 22, 510-517; Xia, C.; Jia, Y.; Tao, M.; Zhang, Q., Physics Letters A 2013, 377, 1943-1947; Pozun, Z. D.; Henkelman, G., The Journal of chemical physics 2011, 134, 224706]. In particular, measurements show a significant increase in efficiency and photo-current in platinum (Pt)-doped PEC cells [Hu, Y.-S.; Kleiman-Shwarsctein, A.; Forman, A. J.; Hazen, D.; Park, J.-N.; McFarland, E. W., Chemistry of Materials 2008, 20, 3803-3805; Mao, A.; Park, N.-G.; Han, G. Y.; Park, J. H., Nanotechnology 2011, 22, 175703; Hsu, Y.-P.; Lee, S.-W.; Chang, J.-K.; Tseng, C.-J.; Lee, K.-R.; Wang, C.-H., Int. J. Electrochem. Sci 2013, 8, 11615-11623; Kim, J. Y.; Magesh, G.; Youn, D. H.; Jang, J.-W.; Kubota, J.; Domen, K.; Lee, J. S., Scientific reports 2013, 3, 2681; Rahman, G.; Joo, O.-S., Materials Chemistry and Physics 2013, 140, 316-322]. In said measurements, Pt was found to increase electron conductivity and therefore was regarded as an n-type dopant. Pt also changes the electrode's morphology, causing smaller grain size, larger surface area, and a more uniform and dense Fe2O3 film, which are thought to aid in charge transport throughout the electrode. In fact, Jae Young Kim et al., using Pt-doped Fe2O3 with a single-crystalline “wormlike” morphology and a cobalt phosphate co-catalyst manufactured the world's highest record for a Fe2O3 PEC cell current density in 2013. All of said studies report an optimum in Pt-doping in the 0.1-4% at. range, yet no study has fully explained why this is the optimal range. In addition, no study has given a complete explanation to why Pt is a successful dopant.
Additional elements, including Si, Ti, Al, Nb, Sn, Cr, Mo, Ni, Mg, Zn, and Ta have been incorporated into α-Fe2O3 as dopants to enhance the photoactivity of hematite [Glasscock, J. A.; Barnes, P. R. F.; Plumb, I. C.; Savvides, N., J. Phys. Chem. C 2007, 111, 16477-16488; Jorand Sartoretti, C.; Alexander, B. D.; Solarska, R.; Rutkowska, I. A.; Augustynski, J.; Cerny, R., J. Phys. Chem. B 2005, 109, 13685-13692; Kleiman-Shwarsctein, A.; Huda, M. N.; Walsh, A.; Yan, Y.; Stucky, G. D.; Hu, Y.-S.; Al-Jassim, M. M.; McFarland, E. W., Chem. Mater. 2009, 22, 510-517; Sanchez, C.; Sieber, K. D.; Somorjai, G. A., J. Electroanal. Chem. Interfacial Electrochem. 1988, 252, 269-290; Ling, Y.; Wang, G.; Wheeler, D. A.; Zhang, J. Z.; Li, Y. Nano Lett. 2011, 11, 2119-2125; Kleiman-Shwarsctein, A.; Hu, Y. S.; Forman, A. J.; Stucky, G. D.; McFarland, E. W., J. Phys. Chem. C 2008, 112, 15900-15907; Liu, Y.; Yu, Y. X.; Zhang, W. D., Electrochim. Acta 2012, 59, 121-127; Ingler, W. B., Jr.; Khan, S. U. M., Thin Solid Films 2004, 461, 301-308; X. Zhang, H. Li, S. Wang, F.-R. F. Fan, and A. J. Bard, J. Phys. Chem. C, 2014, 118 (30), pp 16842-16850].
To the inventors' best knowledge, the doping of the PEC cell electrodes is typically homogeneous, wherein the bulk and the surface of the electrode include similar concentration of the dopant. Since non-uniform photoanodes can significantly lower the photocurrent and the overall device performance, it is important to produce uniform and reproducible doped hematite photoanodes to ensure better device performance [A. Annamalai et al., Solar Energy Materials & Solar Cells 144 (2016) 247-255].
There still exists an unmet need for improved PEC cell electrodes, inter alia, hematite-based electrodes, which would provide enhanced conversion efficiency without significantly increasing the cost or compromising stability of the currently available Pt-doped Fe2O3 material.